A coating is often applied to metal substrates, especially metal substrates that contain iron such as steel, prior to the application of a protective or decorative coating. The coating minimizes the amount of corrosion to the metal substrate, if and when, the metal substrate is exposed to moisture and oxygen. Many of the present coating compositions are based on metal phosphates, and rely on a chrome-containing rinse. The metal phosphates and chrome rinse solutions produce waste streams that are detrimental to the environment. As a result, there is the ever-increasing cost associated with their disposal.
Coating compositions can be applied without chrome rinse solutions. For example, U.S. Pat. No. 3,966,502 discloses post-treating phosphated metals with zirconium-containing rinse solutions. However, this application process is only suitable for use over a limited number of metal substrates, and the generation of metal phosphate waste streams is not alleviated.
U.S. Pat. No. 5,534,082 to Dollman et al. and U.S. Pat. Nos. 5,281,282 and 5,356,490 to Dolan et al. describe non-chrome coating compositions containing a fluoroacid such as fluorotitanic acid, silica, and a water-soluble polymer such as an acrylic acid polymer and/or a polymer with hydroxyl functionality. By heating the silica and fluoroacid, the silica is dissolved, or at least partially dissolved, until the solution is clear. As a result, the silica particles used in these coating compositions are not acid-stable particles. The pH of these compositions is very acidic, and ranges from 0 to 4, preferably from 0 to 1. The coatings compositions enhance the corrosion resistance of steel and galvanized steel substrates.
U.S. Pat. No. 5,938,861 to Inoue et al. describes forming a coating on metal substrates, except aluminum. The coating composition includes an oxidative compound such as nitric acid or hydrogen peroxide, silicate or silicon dioxide particles, and a metal cation, oxymetal anion, or fluorometallate anion of Ti, Zr, Ce, Sr, V, W, and Mo.
EP 1130131A2 to Toshiaki et al. describes a non-chrome coating composition that contains a metallic surface-treating agent, water-dispersible silica, and one or more of a zirconium or titanium compound, thiocarbonyl compound, and a water-soluble acrylic resin. The metallic surface treating agent is selected from a provided list of silane coupling agents that are typically used in the coating industry to improve adhesion between the pre-coating and the decorative coating.
U.S. Pat. No. 5,859,106 to Jones et al. describes a non-chrome coating composition that contains a cross-linked polymer system, which includes a copolymer with acrylic and hydroxyl functionality or the reaction product of an acrylic polymer and a polymer with hydroxyl functionality. A fluoroacid such as fluorozirconic acid or fluorotitanic acid can be added to these compositions. U.S. Pat. No. 5,905,105 to Jones et al. describes a non-chrome coating composition that includes the coating composition described in U.S. Pat. No. 5,859,106 with the addition of dispersed silica and an ammonium carbonate containing a group IVB metal.
There is an interest to develop coating compositions and methods of applying such compositions without producing metal phosphate and chrome waste solutions. It is also preferred, that these coating compositions be effective in minimizing corrosion in a variety of metal substrates because many objects of commercial interest contain more than one type of metal substrate. For example, the automobile industry often relies on metal components that contain more than one type of metal substrate. The use of a coating composition effective for more than one metal substrate would provide a more streamlined manufacturing process.